The invention relates to a metal-halide lamp comprising a discharge vessel with a ceramic wall which encloses a discharge space with an ionizable filling including at least Hg, an alkali halide and CeI3, and which discharge space further accommodates two electrodes whose tips are arranged at a mutual distance EA, and the discharge vessel has an inside diameter Di at least over the distance EA, and the relation EA/Di greater than 5 is met.
A lamp of the type mentioned in the opening paragraph is known from the European patent application No. 96203434.4 U.S. Pat. No. 5,993,453. The known lamp, which combines a high luminous efficacy with acceptable to good color properties (inter alia a general color rendering index Raxe2x89xa745 and a color temperature Tc in the range between 2600 and 4000 K) can particularly suitably be used as a light source for, inter alia, general lighting purposes. As a result of the comparatively small diameter with respect to the electrode distance and hence the discharge arc length, the discharge arc is restrained by the wall of the discharge vessel, and it is attained that the discharge arc has an approximately straight shape. This is very advantageous in connection with the Ce present, since Ce generally has a strong contracting influence on the discharge arc of the lamp. In general, it applies that a discharge arc will exhibit a greater degree of curvature in the horizontal burning position as the degree of contraction of said discharge arc is greater. It has also been found that, as a result of this geometry, the wall of the discharge vessel is subject to such uniform heating that the risk of fracture of the wall of the discharge vessel as a result of thermal stress is very small. It has further been found that said geometry also substantially counteracts the occurrence of spiral-shaped instabilities in the discharge.
By restraining the discharge arc, use is advantageously made of a good thermal conductivity of the ceramic of the wall of the discharge vessel as a means of limiting thermal stresses in the wall of the discharge vessel.
In this description and in the claims, the term ceramic wall is to be understood to mean both a wall of metal oxide, such as sapphire or dense-sintered polycrystalline Al2O3, and a wall of metal nitride, such as AIN. These materials can very suitably be used to manufacture gastight translucent bodies. The light emitted by the known lamp has a color point with co-ordinates (x,y), which differs so much from the color point of the light emitted by a full radiator that it cannot suitably be used for indoor lighting. The collection of color points of a full radiator is commonly referred to as black-body-line (BBL). For indoor lighting purposes, it applies that only light whose color point deviates only slightly from BBL is to be considered as white light. Therefore, in general, it applies for indoor lighting applications that the color point co-ordinates (x,y) deviate maximally (0.03; 0.03) and preferably not more than (0.015; 0.015) from the BBL at the same color temperature Tc.
In the known lamp, use has been made of the insight, which is known per se, that a good color rendering can be achieved if the alkali halide is used in the form of Na-halide as the filling constituent of a lamp, and that during operation of the lamp a strong broadening and reversal of the Na-emission in the Na-D lines occurs. This requires a high temperature of the coldest spot Tkp in the discharge vessel of at least 1100 K (820xc2x0 C.). The requirement of a high value of Tkp excludes, under practical conditions, the use of quartz or quartz glass for the wall of the discharge vessel and compels the use of ceramic for the wall of the discharge vessel.
EP-A-0215524 (PHN 11.485) discloses a metal-halide lamp in which use is made of the above-described insight, and which lamp has excellent color properties (inter alia, general color-rendering index Raxe2x89xa780 and a color temperature Tc in the range between 2600 and 4000 K) and hence can very suitably be used as a light source for, inter alia, indoor lighting. Said known lamp has a relatively short discharge vessel for which applies that 0.9xe2x89xa6EA/Dixe2x89xa62.2, and a high wall load which, for practical lamps, amounts to more than 50 W/cm2. In said application, the wall load is defined as the quotient of the wattage of a lamp and the outer surface of the part of the wall of the discharge vessel located between the electrode tips.
A drawback of this lamp is that it has a relatively limited luminous efficacy.
Metal-halide lamps with a filling comprising not only an alkali metal and Ce, but also Sc, and with a color point which is very close to the BBL, are known per se. However, as a result of its very strong reactive character, Sc proved to be unsuitable for use in a metal-halide lamp having a ceramic lamp vessel.
The invention relates to a measure for obtaining a metal-halide lamp having a high luminous efficacy, which can suitably be used for indoor lighting applications.
To achieve this, the alkali-halide comprises lithium iodide (LiI).
By means of this measure, the lamp emits light with a high luminous efficacy and with a color point which is so close to the BBL that the light emitted by the lamp can be considered to be white light for indoor lighting applications. This is further favorably influenced by the choice of LiI and CeI3 in a molar ratio ranging between 1 and 8. In an advantageous embodiment of the lamp in accordance with the invention, the alkali halide also comprises NaI. Apart from the preservation of a color point which is so close to the BBL that the lamp can be used for indoor lighting purposes, the presence of NaI enables the color point of the lamp to be chosen in a wide range along the BBL. Preferably, LiI and NaI are jointly present in a molar ratio relative to CeI3 ranging between 4 and 10. This enables a lamp to be obtained whose emitted light has a color point whose co-ordinates differ less than (0.015; 0.015) from the BBL, while the color temperature of the light ranges between 3000 K and 4700 K.
Counteracting thermal stresses in the wall of the discharge vessel is further favorably influenced by choosing the wall load to be preferably maximally 30 W/cm2.
A further improvement as regards the control of the wall temperature and of thermal stresses in the wall of the discharge vessel can be achieved by a suitable choice of the wall thickness. The good thermal conductivity of the ceramic wall is further advantageously used if the ceramic wall has a thickness of at least 1 mm. An increase of the wall thickness results in an increase of the thermal radiation through the wall of the discharge vessel, but above all it contributes to a better heat transport from the part of the wall between the electrodes to the relatively cool ends of the discharge vessel. In this manner, it is achieved that the temperature difference occurring at the wall of the discharge vessel is limited to approximately 200 K. An increase of the wall thickness also leads to a decrease of the load on the wall.
Also an increasing ratio EA/Di by increasing EA causes the load on the wall to be limited. In this case, an increasing radiation loss at the wall of the discharge vessel and hence an increasing heat loss of the discharge vessel during operation of the lamp will occur. Under otherwise constant conditions, this will lead to a decrease of Tkp.
To obtain a high luminous efficacy and good color properties, it is necessary for the discharge to contain sufficiently large concentrations of Li, Na and Ce. Since the halide salts are present in excess, this is achieved by the magnitude of Tkp. It has been found that, during operation of the lamp, Tkp assumes a value of at least 100 K. Particularly to attain a sufficiently high vapor pressure of Ce, preferably, a value for Tkt of 1200 K or more is realized.
Also bearing in mind the strong dependence of the Ce vapor pressure upon the temperature, it is not necessary to employ very high values of Tkp, which is favorable for obtaining a long service life of the lamp. In any case, attention should be paid that Tkt is lower than the maximum temperature which the ceramic wall material can withstand for a long period of time.
Further experiments have shown that it is desirable not to exceed 1500 K as the maximum value for Tkp. If Tkp greater than 1500 K, the temperatures and pressures in the discharge vessel assume values such that occurring chemical processes attacking the wall of the discharge vessel give rise to an unacceptable reduction of the service life of the lamp. Preferably, if densely sintered Al2O3 is used for the wall of the discharge vessel the maximum value of Tkp is 1400 K.
In general, a noble gas for ignition of the lamp is added to the ionizable filling of the discharge vessel. The choice of the filling pressure of the noble gas enables the light-technical properties of the lamp to be influenced.